Photonic integrated circuits (PICs) contain multiple optical devices including laser diodes and optical filtering components. Optical filtering components such as Mach-Zhender interferometers (MZIs) and arrayed waveguide routers (AWGRs) are very sensitive to fabrication process variations.
One of the main challenge is to align the wavelengths of lasers and optical filters, since fabrication process variations cause wavelength variations. In general, the wavelengths of lasers are better controlled than those of optical filters. The issue of optical filters, such as MZIs and AWGRs, is that the widths of the waveguides cannot be fabricated precisely enough according to the design, which causes variations of the transmittance wavelength spectra of the filters and resulting mismatches between the transmittance wavelength spectra and the operating wavelengths of lasers.
Due to specifics, scale and precision of manufacturing the photonic integrated circuits, it is difficult to control the widths of the waveguide precisely. However, due to the dimensions of waveguides, even a minor variation of the width can be significant in relative terms. For example, variations for width of 1.4 microns of the waveguide can be variation of 5% of the width of the waveguide, which causes significant wavelength shift from design.
Number of methods addresses this problem by manually and statically tuning the fabricated waveguides. For example, the method described in K. Watanabe et al., “Trimming of InP-based Mach-Zehnder interferometer filling side cladding of high-mesa waveguide with resin,” Electronic Letters, vol. 47, p. 1245, 2011, uses trimming to tune the wavelength of a Mach-Zehnder interferometer, using material with different refractive index surrounding the waveguide to change the effective refractive index in one of the waveguide. This method, however, requires measurement of wavelength characteristics of each interferometer, and trimming each device one by one, adding packaging cost and time. Accordingly, there is a need to reduce dependencies of the wavelength characteristics of an interferometer on the precision of the fabrication process.